The present disclosure relates to the reduction of electromagnetic emissions and interference (“EMI”).
Power supply circuits typically include one or more DC-DC power converters that convert a DC input voltage to a DC output voltage. FIG. 1 shows a conventional power supply system 100 that includes a DC-DC power converter 102 having an input coupled to positive and negative supply rails 105 and 106 to receive power from a power source 101 at a DC input voltage (Vin) and an input current (Iin) and an output coupled to supply power to a load 103 at a DC output voltage (Vout). The converter 102 may provide regulation of the output voltage with changes in load current (Iload). Many DC-DC converters employ switching techniques, which often produce undesirable EMI. To reduce EMI, EMI shunt capacitors 104 may be connected between a shield, metallic case, or base plate 110 of the converter 102 and one or more of the terminals 105-108. Capacitors 104 create a short path back to the origin for energy capacitively or magnetically coupled to the shield 110.
The shield or base plate 110 may be capacitively or directly connected to a neutral terminal 109b (e.g., a chassis ground). Even if directly connected, there will be some amount of resistance and inductance, which is represented by impedance 121 in FIG. 1. Similarly, the negative rail of the power source 101 is directly or capacitively connected at a power distribution panel to the neutral terminal 109a (e.g. earth ground) as shown by impedance 120 in FIG. 1. Although neutral terminals 109a and 109b are connected together, unwanted inductances and resistances often characterize the imperfect connection and are represented by impedance 122 interposed between the two terminals in FIG. 1.
FIG. 2 is a simplified-model of the noise sources in the power converter system 100. The noise current can be represented as a differential-mode current IDM generated by noise current source 112 and flowing between terminals 105 and 106 and a common-mode current ICM generated by noise source 111 and flowing between terminals 105 and 106 and the converter base plate or shield 110 to the chassis or earth ground 109. As shown in FIG. 2 the common mode noise current is capacitively or inductively coupled to terminals 105 and 106 and shield 110. Bypass capacitors 104 shunt some of the differential and common mode noise currents particularly at high frequencies, however, some of the noise current may not be shunted. The circuit path for the differential mode current not shunted by the bypass capacitors 104 is closed through the voltage source 101 and the power distribution system. The circuit path for the common-mode current not shunted by the bypass capacitors 104 is closed through impedances 120, 121, and 122 in the ground system, the voltage source 101 and the power distribution system giving rise to unwanted EMI.